I am trying to find out the most recent security patch that has been installed on a system (e.g. 2015-006). In this case, it is the latest Yosemite (10.10.5). Neither the version nor the build number are helpful here, as security patches do not seem to update them.
All security updates are recorded in receipts folder located here:
ls -ltr /var/db/receipts
If you list the receipts folder, you can see the most recent updates installed.